Potential dependent orientation of acetonitrile on platinum (111) electrode surface studied by sum frequency generation

Sum frequency generation, SFG, vibrational spectroscopy is used to investigate the orientation of acetonitrile (CH3CN) at the Pt(lll) electrode. Acetonitrile is studied as a function of water concentration and electrode potential to determine how these factors affect the adsorbate structure at the interface. The SFG results indicate acetonitrile orients in response to the electrode potential with the C-C bond, C-3 symmetry axis, perpendicular to the surface. The orientation is predominately with the CH3 group toward the metal between 200 and 600 mV and with the CN group toward the metal above 800 mV, (vs NHE, normal hydrogen electrode). The orientation is reversible between 0 and 1400 mV. Addition of water, >0.05x, disrupts the orientation of acetonitrile at the interface. The alignment of acetonitrile along the surface normal is in response to the surface charge at the electrode.